The present disclosure relates to a flash band correction apparatus, a flash band correction method, and an image pickup apparatus which are applied to the case where a flash band is detected which is generated in a frame of image data taken by irradiating a subject with intense light (hereinafter, referred to as “flash”), for example.
In related art, there has been known an image pickup element used for a camera, which uses a rolling shutter system for starting exposure sequentially for each horizontal line (hereinafter, referred to as “line”) and reading an image signal sequentially for each line to output a frame. In an image pickup element with the rolling shutter system, exposure time periods vary by line. Therefore, if a flash or the like is used during a shorter time period than a frame rate during which the image pickup element reads an image signal, a band-like unevenness (flash band) in a brightness level may be generated for each frame of the image displayed. Hereinafter, a brightness level is abbreviated as “level”, and a flash band is abbreviated as “FB”.
FIGS. 18A and 18B are explanatory diagrams showing an example of an FB generated in a frame in related art.
FIG. 18A shows a relationship between a flash and frames output by an image pickup element with a vertical axis representing the number of lines in a vertical direction and a horizontal axis representing time (seconds).
The image pickup element outputs image signals read in a direction from an upper portion toward a lower portion of the frame. Then, a flash is used across the first frame and the second frame.
FIG. 18B is a diagram showing an example of images of the frames.
As shown in FIG. 18A, if the flash is used across the first frame and the second frame, on a line in a lower portion of the first frame, an unevenness of a level is generated due to the flash. On the other hand, for the second frame, before an image signal of a line in a lowermost portion of the first frame is read, an image signal of a line in an upper portion of the second frame is read. For this reason, on the line in the upper portion of the second frame, the unevenness of the level is generated due to the flash.
If the FB is generated in the frames as described above, in the first frame, a lower portion of the image is brightened, and in the second frame, an upper portion of the image is brightened. If the FBs are generated over a plurality of frames, the FBs appear when a moving image is reproduced or when a still image is captured, so the quality of the image deteriorates.
In related art, to suppress the influence exerted on an image by a frame in which the FB is generated, a correction is carried out so that the unevenness of the level is eliminated, or a corresponding frame itself is removed, for example. As a precondition of those countermeasures, a method of detecting existence or nonexistence of the FB has been considered.
As the method of detecting the FB, Japanese Patent Application Laid-open No. 2010-135921 discloses a method of using a first condition for detecting that an area in which the level of pixels is increased exists in a lower portion of a first frame and an upper portion of a second frame. In this method, to detect the FB, a decrease in the second frame in level of the area, the level of which is increased in the first frame, is set as a second condition.
As another method, Japanese Patent Application Laid-open No. 2007-306225 discloses a method of detecting a frame in which an exposure is saturated from among continuous frames taken through an appropriate exposure.